This invention relates to the vapor phase nitration of hydrocarbons. In a particular aspect, this invention relates to an improved process for the vapor phase nitration of methane to produce nitromethane.
The successful vapor phase nitration of methane was first described in U.S. Pat. Nos. 2,161,475 and 2,164,774, issued to G. K. Landen. Both of these patents described the nitration of methane with nitric acid by passing the vaporized reactants through open tubular reactors which were externally heated to maintain a reaction temperature of about 707.degree.-1022.degree. F. for a contact time ranging between about 0.005 and 1 second.
The use of an externally heated tubular reactor is, however, disadvantageous from the standpoint of efficient heat transfer. A considerable length of the reactor is necessarily devoted to preheating the gas to reaction temperature. Also, when the reaction temperature is raised, the exothermic reaction causes a further temperature rise which is difficult to control, especially with externally heated reactors. Moreover, it is difficult to correlate temperature and contact time in such a manner as to maintain optimum relationship between those variables. It will be understood that if the temperature exceeds the optimum range, undesirable side reactions will proceed at a relatively faster rate than that at the optimum temperature. If the temperature falls short of the optimum range, the reaction may cease.
One of the more successful recent attempts at the nitration of methane involved the passage of a mixture of nitric acid and a large excess of methane in the gaseous phase and at an average pressure of 100 psig through a glass tube 3 millimeters in diameter and 180 feet long immersed in a salt bath at a temperature of 410.degree.-430.degree. C. The reaction time was about 1.0 second and the nitromethane yields amounted to approximately 20%, based upon the nitric acid fed to the system. The exceedingly small diameter and great length of reaction tube was required to supply sufficient heat transfer per area per reaction space volume to maintain approximately isothermal conditions during the reaction. The required contact time of 0.1 to 1.0 second necessarily requires an exceedingly high gas velocity and, therefore, unduly high differential pressure across the small reactor tubes. The use of such small tubular reactors of great length is disadvantageous because of the large pressure drop though the reactor. Great difficulty is also often encountered through the blocking and breakage of such reactors.
More recently, R. S. Egly and E. E. Toops, Jr., U.S. Pat. No. 3,378,596 disclosed a process for nitrating alkanes using a jet engine or a rocket engine as a reactor. This process had the advantage of providing very short reaction time of about 50 milliseconds. Also, the bipropellant injectors of such engines are designed to provide rapid atomization of the nitric acid. Egly et al taught the use of several radially-disposed, diametrically opposed inlet injectors which inject a plurality of streams of nitric acid into the flowing stream of methane. Rapid quenching of the reaction was provided by passing the reaction mixture through the exhaust nozzle of the engine. A sudden expansion of the reaction products from the pressure of the reaction chamber, i.e. 60 to 800 psig, to atmospheric pressure immediately terminates the reaction.
Although this process was quite successful, it was energy inefficient because the unreacted methane and oxides of nitrogen had to be recompressed to be efficiently recovered or beneficially utilized. Also, since it operated at essentially atmospheric exit pressure, excessively large and costly coolers, scrubbers, and associated facilities would be required.